1. Field of the Invention
The present invention relates to a system for and a method of removing NOx from exhaust gases emitted from a combustion apparatus such as an internal combustion engine or the like by reacting NOx with NH.sub.3 to produce N.sub.2 and H.sub.2 O.
2. Description of the Related Art
As shown in FIG. 23 of the accompanying drawings, one general NOx removal system for removing NOx from exhaust gases includes an NOx remover 202, which comprises a catalytic converter, for reacting NOx emitted from a combustion apparatus 200 such as an internal combustion engine or the like with NH.sub.3 to produce N.sub.2 and H.sub.2 O. The NOx removal system also has an NH.sub.3 supply controller 208 for introducing NH.sub.3 at a preset rate depending on a load 204 on the combustion apparatus 200 or an output power thereof into a gas passage 206 from the combustion apparatus 200, and an NOx analyzer 210 connected to a discharge passage of the NOx remover 202 through a sampling line L. The NOx analyzer 210 determines the difference between the concentration of NOx in exhaust gases emitted from the NOx remover 202 and a predetermined reference concentration, and the NH.sub.3 supply controller 208 controls the rate of NH.sub.3 introduced into the gas passage 206 based on the determined difference. For details, reference should be made to Japanese laid-open patent publication No. 64-83816.
According to other conventional NOx removal processes, the concentration of NOx in exhaust gases emitted from the combustion apparatus 200 is sampled upstream of the NOx remover 202, and NH.sub.3 is introduced upstream of remover 202 based on the sampled concentration of NOx, or combustion apparatus 200 is measured downstream of the NOx remover 202 for the control of rate of NH.sub.3 to be introduced. The NOx removal system shown in FIG. 23 is more advantageous than those conventional NOx removal processes in that since the concentration of NOx is sampled downstream of the NOx remover 202, the NOx removal system requires no precision measuring instrument, and any discharge of NH.sub.3 from the NOx remover 202 is minimized.
However, the NOx removal system shown in FIG. 23 suffers the following problems:
While the concentration of NOx in emitted exhaust gases is sampled by the NOx analyzer 210, NH.sub.3 contained in the emitted exhaust gases is not measured. Therefore, an excessive introduction of NH.sub.3 cannot be detected, and may be discharged into the atmosphere.
Therefore, the NOx removal system shown in FIG. 23 is premised not to increase the rate of NH.sub.3 to be introduced when the concentration of NOx in the emitted exhaust gases is lower than a certain value, e.g., 80% of a reference value according to the air pollution regulations. If the rate of NH.sub.3 to be introduced were increased to further reduce the concentration of NOx in the emitted exhaust gases, then an increased amount of NH.sub.3 would be discharged into the atmosphere.
The NOx removal system shown in FIG. 23 may not necessarily minimize the amount of both NOx and NH.sub.3 discharged into the atmosphere.
Another drawback of the NOx removal system is that when the purifying efficiency of the NOx remover 202 is lowered to the point where the amount of emitted NOx exceeds a predetermined level, the rate of introduced NH.sub.3 is automatically increased even though it is already sufficient, and hence the amount of NH.sub.3 emitted into the atmosphere is increased.
The above drawback may be eliminated if an NH.sub.3 analyzer is added downstream the NOx remover 202 for monitoring emitted NH.sub.3 so that the NH.sub.3 supply controller 208 controls the rate of introduced NH.sub.3 in a manner to keep the concentration of NH.sub.3 at a low level and lower the amount of emitted NOx. However, adding the NH.sub.3 analyzer will increase the size of the NOx removal system and require the NH.sub.3 supply controller 208 to have a more complex control circuit. Even if two analyzers, i.e., an NOx anaLyzer and an NH.sub.3 analyzer, are added, since they usually have difference responses, it is difficult for the NH.sub.3 supply controller 208 to control the NOx and NH.sub.3 concentrations highly accurately.
NOx analyzers are usually CLD or NDIR analyzers that are highly expensive and have slow responses which do not allow the rate of introduced NH.sub.3 to be highly accurately controlled.
Another conventional NOx removal system has an NOx catalyst and an oxidizing catalyst which are disposed in the exhaust system of a diesel engine, and an apparatus for introducing urea into the exhaust system upstream of the NOx catalyst. The introduced urea produces NH.sub.3 in the NOx catalyst.
In this NOx removal system, the rate of urea to be introduced is controlled according to mapping information (representative of the relationship between engine operating conditions and NOx concentrations) of NOx which is stored in a microprocessor, and the temperature of the NOx catalyst. However, the NOx concentration or the amount of NOx determined from the mapping information may deviate from an actually emitted amount, with the result the NOx removal system may fail to remove NOx efficiently from the exhaust gases, and the introduced urea or NH.sub.3 decomposed therefrom may be emitted from the exhaust system.
Japanese laid-open patent publications Nos. 4-358716 and 7-127503 disclose NOx removal systems in which HC is introduced instead of NH.sub.3, an NOx sensor is attached downstream of an NOx catalyst, and the rate of HC to be introduced is controlled by a signal from the NOx sensor.
These systems have a poor NOx reduction efficiency because HC is used as a reducing agent. Even if an efficient catalyst temperature range is selected, the NOx reduction efficiency ranges from 40% to 60% at most, and almost half of the introduced amount of HC is discharged. Furthermore, the efficient catalyst temperature range is very limited, i.e., it is 400.degree. C..+-.50.degree. C. Consequently, these systems may possibly frequently cause HC to be discharged beyond its emission control level in applications where the temperature of exhaust systems vary widely, e.g., automobile engines. To avoid such a possibility, it is necessary to add catalyst cooling and heating devices to the system.
Japanese laid-open patent publication No. 5-113116 reveals a system similar to the above systems except that a basic HC rate determined in advance depending engine operating conditions is corrected on the basis of a signal from the NOx sensor. This system, however, also suffer the above shortcomings.